1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus.
2. Description of Related Art
A conventional example of an image forming apparatus such as an electro-photographic printer includes an image forming apparatus in which a sheet in which an image is formed on one surface is reversed and conveyed to an image forming portion again, thereby forming the image on back side of the sheet. Such an image forming apparatus has a sheet conveying apparatus which reverses the sheet in which the image is formed on one surface and conveys it to the image forming portion again.
In such a conventional sheet conveying apparatus, when the sheet is conveyed to the image forming portion again, the sheet may be skew-fed during conveyance. When the image is formed on back side, the sheet and the image may be shifted. This is because when the image is formed on a second surface (back side), a conveying path until the image is formed on the sheet is longer than that of a first surface, and thus the sheet is slightly shifted during conveyance depending on differences in eccentricity of various rollers and welding pressure or differences in resistance of the sheet conveying surface.
In order to prevent such a shifting of the sheet, it is necessary to adjust the position of the sheet so that the image and the sheet are matched until the image is formed on the second surface after forming the image on the first surface.
As an example of the method for adjusting the position of the sheet, there is listed a technique in which a reference guide is disposed at an end portion in a direction perpendicular to the sheet conveying direction of a reconveying passage (hereinafter referred to as the width direction) which conveys the sheet to the image forming portion again. There is a method in which alignment in the width direction of the sheet (hereinafter referred to as lateral register correction) is performed by conveying the sheet while pressing it against the reference guide (Japanese Patent Application Laid-Open (JP-A) No. 2000-233850).
FIG. 15 is a top perspective view illustrating the structure of the reconveying passage of the conventional sheet conveying apparatus having a lateral register correction portion that performs lateral register correction of the sheet by such a reference guide.
The lateral register correcting portion 23 has a reference guide 24 that includes a reference surface 24a, a pair of skew conveying rollers 11A that includes a skew conveying roller 11 and a skew conveying roller bearing (not shown), and a conveying lower guide 27. In this regard, the skew conveying roller bearing is disposed so as to be inclined toward the reference surface 24a at about 3 to 15° and has a drum-shaped form. Two reference surfaces 24a is rubbed by the sheet end portion when the sheet is fed. Thus, a plurality of reference pins 24h1 and 24h2 made of stainless steel having a cylindrical-shaped form are disposed in (pressed into) the reference surface in order to enhance them.
Subsequently, lateral register correcting operation of the lateral register correcting portion 23 having such a structure will be described.
As shown in FIG. 15(a), the sheet S in which an image is formed on one side is conveyed from a conveying roller 3g provided at upstream of the lateral register correcting portion 23 to the lateral register correcting portion 23. When the sheet S reaches the pair of skew conveying rollers 11A, the sheet S is then nipped and conveyed by the pair of skew conveying rollers 11A. Then, the sheet S is conveyed while it is drawn to the side of the reference surface 24a by the pair of skew conveying rollers 11A.
Subsequently, when the rear end of the sheet S is passed through the conveying roller 3g, the resistance of the conveying lower guide 27 allows the sheet to rotate so as to bring the rear end of the sheet close to the side of the reference surface 24a as shown in FIG. 15(b). Then, the sheet S abuts against a reference pin 24h1 by the rotation and the reaction force allows the edge of the sheet to rotate in a direction bringing it close to the side of the reference surface 24a. 
As a result, the sheet S is aligned with the position along the reference surface 24a as shown in FIG. 15(c). Then, the sheet S that is thus subjected to lateral register correction by the lateral register correcting portion 23 is reconveyed to an image forming portion (not shown) via the intermediate roller 3d. 
As described above, in the lateral register correcting portion 23, the resistance of the pair of skew conveying rollers 11A and the conveying lower guide 27 allows the sheet S to rotate and then the reference pin 24h1 serves as a pivot point to reverse the sheet S in order to move it along the reference surface 24a. As for the lateral register correction method, a conveying distance from the time the rear end of the sheet passes through the conveying roller 3g until it moves along the reference surface 24a is short and thus the efficiency of alignment (skew feeding correction) is good. Therefore, even when the duplex sheet is fed, a slight shifting of the sheet is corrected, thereby allowing the sheet to be reconveyed.
As shown in FIG. 16, the reference pins 24h1 and 24h2 disposed in the reference surface 24a of the reference guide 24 is projected out by only an 1 width toward the reference surface 24a. This prevents the sheet from abutting against the reference surface 24a directly. This can prevent the reference surface 24a from being worn out by sliding along the side edge of the sheet and further can prevent paper jams from occurring by the resistance rise in the sheet and the reference surface caused by wear when the sheet is fed (refer to Japanese Patent No. 3092986).
However, in such conventional sheet conveying apparatuses, shifting of the sheet may occur in the width direction (thrust direction) during conveyance of the sheet. When a lot of sheets are fed, the sheet is shifted by only a sheet width −X by variations of the attached position of a side regulating plate that regulates the side edge position of the sheet and variations of the alignment of each conveying roller, thereby causing the engine to be conveyed to the reference guide.
When the sheet is shifted by only the width −X in the width direction and conveyed to the reference guide, the sheet side edge directly abuts against a reference guide 24 in a sheet introducing area J at the upstream side by the reference pin 24h1 that serves as a pivot point shown in FIG. 16. In this case, the sheet side edge slides along the reference guides 24 and the sliding of the sheet causes wearing of the reference guide 24. Further, the wearing leads to sliding resistance between the sheet side edge and the reference guide 24, thereby causing paper jams easily. This becomes more remarkable as the duration of use becomes longer.
FIG. 17 is a diagram describing a state when the sheet S is shifted by the width −X and conveyed to the reference guide 24, FIG. 17(a) is a plain view at the time, and FIG. 17(b) is a cross-sectional view when a state at the time is viewed from the downstream in a sheet conveying direction. In this regard, the cross section of the reference guide 24 has a U-shaped form as shown in FIG. 17(b).
Here, in FIG. 17(a), a sheet introducing portion 24b is provided at the upstream side of the reference guide 24 and guides the sheet S being conveyed in the direction of an arrow B by the conveying roller 3g and a conveying lower guide 127 is arranged in parallel to the reference guides 24 and includes the undersurface (bottom surface) of the reconveying passage. In this regard, a controlling portion member 127a that controls the vertical movement of the sheet S is provided at the upper part of the conveying lower guide 127.
The lateral register correcting portion 23 has two (a plurality of) conveying rollers 3g that conveys the sheet. Here, when the conveying roller 3g is one, the edge of the sheet is easily rotated during conveyance. In this case, when the edge of the sheet is conveyed, the skew feeding of the sheet maybe come great or the sheet may be shifted in the width direction. Therefore, two or more conveying rollers 3g are disposed in the width direction to prevent the sheet from rotating in such a manner.
In FIG. 17(b), a path interval PT4 is an interval in a vertical direction of the reference guide 24, the conveying lower guide 127, and the controlling portion member 127a in an entire area in a sheet conveying direction of the lateral register correcting portion 23. When the path interval PT4 is too wide, a path difference in a height direction is caused in the width direction of the sheet until the edge of the sheet is discharged from the lateral register correcting portion 23 and nipped by a roller (not shown) at downstream. When the path difference in the height direction is thus caused, skew feeding of the sheet and shifting in the width direction are easily generated at downstream of the lateral register correcting portion 23. Then, a path interval of the reference guide 24 is about 2 mm.
Here, as shown in FIG. 17(a), the sheet S shifted by only a width XT in the width direction and conveyed abuts against the sheet introducing portion 24b and is then guided in the direction of an arrow Rv by the sheet inserting portion 24b. Thereafter, the edge of the sheet is moved in the width direction to a reference line L used as a standard of the width direction. A gap is provided among the reference guide 24 of the conveying lower guide 127, the bottom surface on the opposite side in the width direction, and the sheet S. The side abutted against the sheet introducing portion 24 and the edge portion of the sheet on the opposite side are deflected by a width of the gap, which allows the sheet S to rotate in the direction of the arrow Rv.
However, when the width XT that is a shifted portion of the sheet S in the width direction is large, the deflection amount of the sheet S becomes larger. Accordingly, the elasticity of the sheet S becomes stronger. In this case, unless the reference guide side of the sheet S is not deflected as shown in FIG. 17(b), the sheet S cannot be guided, resulting in paper jams. Even if the edge of the sheet can be guided while the sheet S is deflected, when the path interval PT4 is narrow, it is difficult to deflect the sheet S.
On the other hand, when the reference guide side of the sheet S is thus deflected, force acting on the sheet introducing portion 24b of the sheet side edge is strengthened by the elasticity of the sheet S due to the deflection. Thus, rubbing and wearing of the sheet introducing portion 24b are easily caused. Further, resistance between the sheet side edge and the sheet introducing portion 24b becomes larger as the force acting on the sheet introducing portion 24b by the sheet side edge is stronger. Then, the resistance becomes conveying resistance, thereby causing paper jams.
For that reason, the sheet can be easily deflected in the sheet conveying direction between a sheet distance ST from the conveying roller 3g at the upstream side to the edge of the sheet and it is necessary to have a structure in which the edge of the sheet is rotatable in the direction of the arrow Rv.
On the other hand, FIG. 18 is a diagram illustrating when the pin 24h3 is disposed in the sheet introducing portion 24b so as to have a distance XP from the pin 24h1 provided in the reference surface 24a. Here, as shown in FIG. 18(a), when the sheet is shifted to the reference surface 24a between the distance XP and conveyed, the sheet introducing portion 24b slides along the edge of the sheet. This causes rubbing and wearing.
As shown in FIG. 18(b), when the sheet S is shifted by only a width XJ that abuts the reference pin 24h3 and conveyed, an angle JK formed by a diameter R of the reference pin 24h3 and the sheet side edge becomes acute (e.g. 45° or more), thereby causing paper jams.
FIG. 19 is a structure that a part of the sheet introducing portion 24b includes a U-shaped metallic member 170 having a path interval T such as SUS and galvanized steel sheet. The path interval T of the U-shaped metallic member 170 is set to about 2 to 3 mm taking into consideration the height and the conveying performance of the lateral register correcting portion in itself. Since the path interval T of the metallic member 170 is narrow, each corner of the inner wall has a form R of about 0.5 to 1 mm considering the durability of the metallic mold.
Here, as shown in FIG. 19(a), when the sheet S is shifted by the width −X and conveyed in the direction of the arrow B, the metallic member 170 guides the edge of the sheet toward the direction of the arrow Rv so as to move along the sheet introducing portion 24b. 
However, each corner of the inner wall of the metallic member 170 has the form R and thus the sheet end portion is curled up along the form R while the metallic member 170 guides the edge of the sheet to the reference surface 24a as shown in FIG. 19(b). As a result, the metallic member 170 cannot guide the edge of the sheet toward the direction of the arrow Rv and the curled edge of the sheet is folded. Then, paper jams are caused by the conveying resistance of the sheet S.
Even if the sheet S is guided to the reference surface 24a and conveyed in a state that the shifting of the sheet S is slight and the edge of the sheet is not curled up, sliding surface along the sheet side edge of the metallic member 170 is scratched and worn as the duration of use becomes longer. When the metallic member 170 is scratched by the sheet side edge, the sliding resistance along the sheet side edge maybe increased. Finally, paper jams are caused by the scratched metallic member 170 and resistance between the worn surface and the sheet side edge.
When each corner of the metallic member 170 has a rectangular form instead of the form R without taking into consideration the metallic mold strength, curing of the edge of the sheet can be prevented. However, as the duration of use becomes longer, each corner portion of the metallic member 170 is significantly scratched and worn by the sheet side edge to be guided. As a result, paper jams are caused by sliding resistance.
For that reason, it is important to reduce the force acting on the side edge of the sheet introducing portion 24b as well as scratching and wearing of the sheet introducing portion 24b by the elasticity of the sheet when the edge of the sheet is guide in the direction of the arrow Rv. Here, if the sheet side edge can be easily rotated in the direction of the arrow Rv, the force acting on the side edge of the sheet introducing portion 24b by the sheet side edge can be reduced.
JP-A No. 2004-299856 describes a structure that includes a first reference guide surface 12a that abuts against the side edge of letter-size and A4-size sheets and a second reference guide surface 12b that is a side edge standard of the sheets S of an executive-size and a B5 size. Further, in order to adapt to an A5-size sheet, namely a sheet S1 having a width narrower than that of A5 paper, the third reference guide surface 12c is included. The first reference guide surface 12a, the second reference guide surface 12b, and the third reference guide surface 12c are respectively formed so that they are shifted in the width and thickness directions of the sheet to be conveyed.
Here, when the sheet is conveyed while the side edge of the sheet abuts against the first reference guide surface 12a, a supporting surface 12a1 which supports an end portion of the sheet is positioned higher than a guiding surface 20a of a conveying lower guide 20 which guides the other end of the sheet. In the structure of JP-A 2004-299856, the sheet is conveyed in a downstream direction in the condition where the height of the end portion of the sheet is different from that of the other end, and thus there is a difference in height between both ends of the sheet. As a result, the accuracy of the sheet position is reduced.